My invention is adapted for use in a variable-assist power steering system such as that shown in U.S. Pat. Nos. 4,561,521 and 4,570,736. It may be used as well in a steering system such as that shown in U.S. Pat. No. 4,877,099. These prior art patents are assigned to the assignee of my invention.
Each of the steering systems described in these patents includes a rotary steering valve that controls fluid flow from the pressure side of a power steering pump to the return side, thereby controlling pressure distribution to a double-acting fluid motor which, in turn, is connected through a steering linkage to the dirigible wheels of a vehicle. The pump is a positive-displacement, constant-flow pump.
The steering valve comprises an inner valve member and a surrounding sleeve valve element which are formed with cooperating rotary valve lands. The sleeve valve element is connected to the pinion of a rack-and-pinion steering gear assembly, the fluid motor being adapted to apply a pressure assist to the steering rack.
Steering wheel torque causes rotary displacement of the inner valve member relative to the sleeve valve element. The amount of the relative rotary displacement is equal to the torsional twist of a torsion bar connection between the steering torque input shaft and the pinion of the steering gear assembly. Because of the constant flow delivered by the pump, displacement of the inner valve member relative to the sleeve valve element causes a steering pressure to be developed on one side of the fluid motor or the other depending upon the direction of the torsional displacement of the torsion bar connection.
The rotary valve lands provide a variable flow area for the fluid delivered from the pump. The steering pressure on one side of the fluid motor or the other is increased in accordance with the reciprocal of the square of the effective flow area across the valve lands, assuming that the power steering pump delivers a constant flow.
In the case of a variable-assist power steering system, a set of high-speed valve lands is provided in addition to a set of low-speed valve lands so that the flow metering area available when both sets of valve lands are operable is greater. This condition exists during high-speed operation. Therefore, at high speeds of operation, a greater degree of relative valve land displacement is required to produce any given steering pressure.
In a system of the kind shown in the ,521 prior art reference patent referred to above, fluid is distributed to the sets of high-speed valve lands through an actuator valve when the vehicle speed exceeds a predetermined value. The actuator valve is a solenoid-operated valve wherein the solenoid forces act directly on the valve element itself. In the case of the system shown in the '736 patent, an electric actuator controls the movement of the actuator valve, the actuator being in the form of a stepper motor which adjusts the valve linearly in one direction or the other depending upon the direction of rotation of the stepper motor. The stepper motor is controlled by an electronic control module that receives a vehicle speed signal.
An electronic actuator in the form of a stepper motor is relatively complex and costly and, in some instances, provides packaging problems in a vehicle having limited space for vehicle accessories in the transmission and engine compartment. In the case of a solenoid-actuated valve of the design shown in the '521 patent, the solenoid forces act directly on the valve, thus making it necessary to use a large solenoid to produce the necessary forces to balance the flow and spring forces on the valve.